Estrogen given to a properly primed mammal is a trigger for embryos to implant in the uterus and continue development. Data collected during the previous grant period, together with reports in the literature, suggest that estradiol may act indirectly via the uterine stroma to promote cleavage of heparan sulfate proteoglycans (HSPG) from the epithelial basolateral surface. This leads to a rounding of the epithelial cells and turnover of their anionic surface to the blastocyst. To test this idea we will (1) characterize the changes in the dominant anionic glycoconjugates on the epithelial surface during implantation, (2) examine the changes during implantation in stromal cell secretions, particularly lytic enzymes, (3) determine the influence of stromal secretions on epithelial ultrastructure and surface anionic glycoconjugates in epithelial-stromal combinations in culture, and (4) identify those epithelial surface anions that bind to and/or block adhesive sites on the blastocyst surface in culture.